Organic light-emitting diode (OLED) display devices can be classified as either small molecule or polymeric type (sometimes called PLED). Typically, processing of OLED small molecule based devices can be very difficult using vapor deposition processes in conjunction with masking technologies. PLED based device processing has been achieved using various solution based processing techniques and some dry based processing, although with difficulty.
While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm ) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
Ink jet recording or printing systems are commonly used to create high-resolution patterns on a substrate. In a typical ink jet recording or printing system, ink droplets are ejected from a nozzle towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a functional material or functional material, such as a dye or pigment or polymer, and a large amount of solvent. In conventional inkjet printing systems, the liquid ink droplets are ejected from the nozzle using pressure pulses generated by an oscillating piezoelectric crystal or by heating the nozzle to generate an ink droplet resulting from bubble formation or from ink phase change. The solvent, or carrier liquid, typically is made up of water, an organic material such as a monohydric alcohol, a polyhydric alcohol or mixtures thereof. There can be many additives in the system aimed at preserving the pixel integrity upon deposition to the receiver. Such materials may be surfactants, humectants, biocides, rheology modifiers, sequestrants, pH adjusters, and penetrants among others. Such materials are necessary due to the high solvent loads in conventional ink formulations. More recently, the inkjet printing method has been used to make electroluminescent display devices.
U.S. Pat. No. 5,972,419 discloses a method of making a multicolor display device, comprising a transparent substrate, electroluminescent materials deposited via an inkjet printing mechanism into wells that are defined by masks produced via a lithographic technique. There is a problem with this invention in that the inkjet printing compositions which contain electroluminescent materials have high solvent loads to be used in conventional ink jet printers. These solvents typically are toxic, for example xylene. Also the substrate needs to be dried after printing to remove the solvent, thereby increasing the manufacturing time. Further, due to solvent content, there is a necessity to have masks fabricated to produce the array of pixels, which increases process time and complexity potentially decreasing device yield. These masks are necessary so that the deposited liquid containing electroluminescent material solutions from adjacent pixels do not contact each other.
U.S. Pat. No. 5,869,350 discloses a method of making a light emitting diode, comprising a glass substrate, where an electroluminescent material is spin-coated onto the substrate using a 1 wt % electroluminescent polymer-tetrahydrofuran solution. There is a problem with this invention in that the spin-cast technique uses very high solvent loads requiring removal of toxic organic solvents. Also, with systems containing low solids weight percent in liquid solutions, a significant dry time is necessary which can increase manufacturing time. Further, to make full color displays, it would be necessary to use masks to produce pixels of various colors thereby increasing process complexity and potentially decreasing device yield. Moreover, spin coating techniques tends to produce films with varying thickness along the length of the device, which is unacceptable.
U.S. Pat. No. 5,851,709 discloses a method to produce electroluminescent displays by providing a suitable substrate, and donor sheets that contain organic light emissive materials. A device is used to transfer the organic light emissive materials onto the substrate in pixel patterns in order to form the display. There is a problem with this technology in that the donor sheet can be complex consisting of several layers and must be fabricated prior to the transfer step therefore adding additional steps to the manufacturing process. Also, handling of the donor sheet is difficult because water may degrade or adsorb into the layers potentially causing process difficulties. Further, the transfer device typically uses a laser that heats a portion of the donor sheet in order to transfer the organic light emissive material potentially causing material degradation due to high temperatures at the point of irradiation. Also, the donor sheet must be affixed in registered, close proximity to the substrate to produce the appropriate pixels increasing the processing complexity.
There are alternate technologies that are available in the prior art, that eliminate this problem by using gaseous propellants. For example, Peeters et al., in U.S. Pat. No. 6,116,718, disclose a print head for use in a marking apparatus in which a propellant gas is passed through a channel, the functional material is introduced controllably into the propellant stream to form a ballistic aerosol for propelling non-colloidal, solid or semi-solid particulate or a liquid, toward a receiver with sufficient kinetic energy to fuse the marking material to the receiver. There is a problem with this technology in that the functional material and propellant stream are two different entities and the propellant is used to impart kinetic energy to the functional material. This can cause functional material agglomeration leading to nozzle obstruction and poor control over functional material deposition. Another problem with this technology is that when the functional material is added into the propellant stream in the channel it forms a non-colloidal ballistic aerosol prior to exiting the print head. This non-colloidal ballistic aerosol, which is a combination of the functional material and the propellant, is not thermodynamically stable. As such, the functional material is prone to settling in the propellant stream, which in turn, can cause functional material agglomeration leading to nozzle obstruction and poor control over functional material deposition.
Technologies that use supercritical fluid solvents to create thin films are also known. For example, R. D. Smith in U.S. Pat. No. 4,734,227, issued Mar. 29, 1988, discloses a method of depositing solid films or creating fine powders through the dissolution of a solid material into a supercritical fluid solution and then rapidly expanding the solution to create particles of the functional material in the form of fine powders or long thin fibers which may be used to make films. There is a problem with this method in that the free-jet expansion of the supercritical fluid solution results in a non-collimated/defocused spray that cannot be used to create high-resolution patterns on a receiver. Further, defocusing leads to losses of the functional material.
Huck et al., in WO 02/45868 A2, discloses a method of creating a pattern on a surface of a wafer using compressed carbon dioxide. The method includes dissolving or suspending a material in a solvent phase containing compressed carbon dioxide, and depositing the solution or suspension onto the surface of the wafer, the evaporation of the solvent phase leaving a patterned deposit of the material. The wafer is prepatterned using lithography to provide the wafer with hydrophilic and hydrophobic areas. After deposition of the solution (or suspension) onto the wafer surface followed by the evaporation of the solvent phase, the material (a polymer) adheres to one of the hydrophobic and hydrophilic areas. The solution (or suspension) is deposited on the wafer surface either in the form of liquid drops or a feathered spray.
This method is disadvantaged because deposition using a feathered spray requires that the wafer surface be prepatterned prior to deposition. Hence, direct patterning of the wafer surface is not possible because of the diverging profile (feathered) of the spray. Additionally, a wafer surface that has not been prepatterned cannot be patterned using this method. This method also requires time for drying so that the solvent phase of the liquid drops (or feathered spray) can evaporate. During the time associated with solvent phase evaporation, the solvent and the material can diffuse from one pixel to next (for example, into the surface or along the surface) degrading the desired pattern.
A different approach for creating high-resolution patterns is needed. A technique that eliminates the issues with solvent management would be advantageous. There is also a need for a technology that permits high speed, accurate, and precise deposition of a functional material, more specifically, hole transporting material on a substrate to create display devices. There is also a need for a technology that permits high speed, accurate, and precise patterning of a substrate that can be used to create high-resolution patterns on a receiver to form electroluminescent displays. There is also a need for formulations that permit high speed, accurate and precise deposition of a functional material, more specifically, electroluminescent polymeric material on a substrate to create imaging and display display devices.